In recent years, electromagnetic waves in the Terahertz frequency band that are between light and radio waves (in other words, electromagnetic waves having a frequency of 1012 Hz, and a wavelength of 30 μm to 1 mm, hereafter referred to as THz waves), are gaining attention as electromagnetic waves that directly reflect information about an object. In order to detect THz (Terahertz) waves, technology of a bolometer type infrared detector having thermal isolation structure is used. As related technology, there is a 2-dimensional bolometer type THz wave detector that is disclosed in Japanese Patent Application KOKAI Publication No. 2008-241438 and SPIE literature (Oda, et al.; Proceedings of SPIE, Vol. 6940 (2008), pp. 69402Y-1 to 69402Y-12).
FIG. 6 illustrates the pixel structure of the bolometer type THz wave detector of the technology mentioned above. The bolometer type THz wave detector 1 comprises a circuit substrate 2 in which a read-out integrated circuit 2a and the like are formed, a reflective film 3 that is formed on that circuit substrate 2 and that reflects incident THz waves, a contact 4 that is electrically connected to the read-out integrated circuit 2a, and a first protective film 5 that is formed on the reflective film 3 and the read-out integrated circuit 2a. Moreover, a support portion 13 that comprises a second protective film 6, a third protective film 8, an electrode wiring 9 and a fourth protective film 10 is formed on the contact 4. The electrode wiring 9 that is included in the support portion 13 is electrically connected to the read-out integrated circuit 2a via the contact 4. This support portion 13, together with forming a space underneath a diaphragm-shaped temperature detecting portion 14, which comprises the second protective film 6, a thin bolometer film 7, the third protective film 8, the fourth protective film 10 and an absorption film 11, forms an air gap 15 between the temperature detecting portion 14 and the first protective film 5 (circuit substrate 2). The electrode wiring 9 is electrically connected to both end sections of the thin bolometer film 7. An eave-like member 12 for absorbing incident THz waves is formed around the temperature detecting portion 14. Absorption film 11 is also formed on this eave-like member 12.
Features of the pixel structure in this related technology are that the gap (air gap) between the reflective film 3 and the temperature detecting portion 14 and the gap between the reflective film 3 and the eave-like member 12 are both set based on an infrared wavelength, and the sheet resistance of the temperature detecting portion 14 is set to 10 Ω/square to 100Ω/square. As a result, with the basic structure of a bolometer type infrared detector array sensor being maintained, an improvement of THz wave sensitivity, for example a nearly 6-fold increase in the sensitivity at 3 THz (100 μm wavelength), is achieved by simply adding an adsorption film 11 (Oda, et al.; Proceedings of SPIE, Vol. 6940 (2008), pp. 69402Y-1 to 69402Y-12).
In the pixel structure of the 2-dimensional bolometer type THz wave detector of the related technology above, when the gap between the reflective film 3 and the absorption film 11 on the temperature detecting portion 14, and the gap between the reflective film 3 and the absorption film 11 on the eave-like member 12 are set to 1.5 μm (β1% occupancy) and 3.0 μm (β2% occupancy), respectively based on an infrared wavelength, the absorptance of that pixel structure of Terahertz waves is dramatically improved as described above when compared with the case when there is no absorption film 11, however, the absorptance still remains a small value. This phenomenon is explained below using a model calculation.
Typically, the sensitivity of a 2-dimensional bolometer type THz wave detector is proportional to the overall absorptance of electromagnetic waves (THz waves) by a temperature detecting portion and an eave-like member. The pixel structure in FIG. 6 is divided into a temperature detection section (diaphragm) 14 (area I) and an eave-like member 12 (area II) as is simply illustrated in FIG. 7. Here, the gap between the temperature detecting portion (diaphragm) 14 (area I) and the reflective film 3, and the gap (air gap) between the eave-like member 12 (area II) and the reflective film 3 are taken to be d1 and d2, respectively, and the occupancy (%) of each area I and II is taken to be β1 and β2, respectively. By taking the sheet resistance of the absorption film 11 that is formed on the temperature detecting portion 14 and the eave-like member 12 to be σS, and the sheet resistance of the reflective film 3 to be σr, and using the equation given in P. A. Silberg (Journal of Optical Society of America, vol. 47 (1957), pp. 575-578), the overall absorptance (η) of the electromagnetic waves (THz waves) by the 2-dimensional bolometer type THz wave detector is given by Equation 1 below.Overall absorption: η=β1A(λ,d1)+β2A(λ,d2)  (Equation 1)where
      A    ⁡          (              λ        ,                  d          1                    )        =            4              Dn        2              ⁢          {                                    [                                                                                                      f                      s                                        ⁡                                          (                                                                        f                          r                                                +                        1                                            )                                                        2                                /                                  n                  2                                            +                              f                r                                      ]                    ⁢                      sin            2                    ⁢          θ                +                              (                                          f                r                            +                              f                s                                      )                    ⁢                      cos            2                    ⁢          θ                    }      D=[(fr+1)(fs+1)/n2+1]2 sin2θ+[(fr+fs+2)2/n2] cos2θ
fr=120π/σr 
fs=120π/σs 
θ=2πnd1/λ
FIG. 8 illustrates the relationship between the overall absorptance of THz waves and the sheet resistance of the absorption film at THz wavelengths λ=70, 100, 150 and 300 μm. In the cases illustrated in FIG. 8, the occupancy rates of area I and area II are 50% and 30%, respectively. As can be understood from FIG. 8, in the case of the related technology, the maximum values of the overall absorptance at the THz wavelengths of 70 μm, 100 μm, 150 μm and 300 μm are low values of 22%, 17%, 12% and 6%, respectively.
The absorptance of silicon nitride is disclosed in lecture materials by Q. Hu et al. (“Real-time THz Imaging Using Quantum-cascade Lasers and Focal-plane Array Cameras”, 2nd International Workshop on Quantum Cascade Lasers, Sep. 6-9 (2006)). The absorptance for silicon nitride is illustrated in FIG. 9. As can be understood from FIG. 9, silicon nitride is nearly transparent at a THz wavelength of 50 μm or greater. Therefore, in this wavelength area, by using the equation in the SPIE literature (Oda, et al.; Proceedings of SPIE, Vol. 6940 (2008), pgs. 69402Y-1 to 69402Y-12) (an absorptance by an interference effect) it is possible to calculate the absorptance of the thermal isolation structure of a bolometer type THz wave detector. However, at a wavelength of 50 μm or less, the absorptance of silicon nitride itself become large, so it is necessary to keep in mind that the absorptance by the silicon nitride itself, which is the main structural material of the temperature detecting portion, contributes to the absorptance rather than the contribution to the absorptance by an interference effect.